1. Field of the Invention
The present invention is directed to thermosensitive recording materials and more particularly, to those which are prepared by dispersing a generally colorless or light colored dye precursor with a dispersing agent, a color developer capable of forming a color of said dye precursor in response to heating with said dispersing agent and a sensitizer together with a dispersing agent, in a finely divided particulate state, mixing each dispersion with a binder, coating the mixture onto a support and drying.
2. Discussion on Related Arts
Thermosensitive recording materials are basically prepared by the following procedures. That is, firstly, a dye precursor, a dye developer and a sensitizer independently or as a mixture thereof is dispersed in water containing a dispersing agent therein and the dispersion is treated with a sand mill, a ball mill, etc. to finely divide those agents, whereby a dispersion composed of finely divided particles is prepared.
Next, a finely divided dye precursor dispersion, a finely divided dye developer dispersion and a finely divided sensitizer dispersion as well as a binder, etc. are thoroughly mixed with stirring. The thus obtained thermosensitive coating liquid is uniformly coated onto a base material such as paper, etc. The system is dried to form a thermosensitive color forming layer. Then, the surface is rendered by supercalendering to give a thermosensitive recording material.
The following properties are required for the dispersing agent and binder used for the aforesaid thermosensitive coating liquid.
(1) The dispersing agent has good compatibility with the binder. And upon mixing a dispersion composed of the dye precursor, color developer and sensitizer with the binder, they are uniformly mixed.
(2) A viscosity of the dispersing agent is not excessively high so as to cause reduction of grinding efficiency of a sand mill, etc. Further they have minimized foamability so that defoaming is rapidly realized.
(3) They have no undesired interaction with the dye precursor, dye developer or sensitizer.
(4) The binder has a high adhesive strength property and shows the sufficient picking strength when formed into a thermosensitive recording material.
Such a dispersing agent and binder are very limited; in many cases, polyvinyl alcohol is used as a main dispersing agent and binder (for example, Japanese Patent Unexamined Publication No. A22792/1984). In some case, for the purpose of further improving the grinding efficiency, a finely divided dye precursor dispersion, a finely divided dye developer dispersion and a finely divided sensitizer dispersion are prepared using a dispersing agent such as styrene-maleic anhydride copolymer which is a water soluble high molecular substance and polyvinyl alcohol is then mixed with these dispersions and the resulting mixture is used.
In the construction described above, however, thermofusible substances are adhered around a heat generating part of the thermal head mounted to a printer, etc., upon color formation of the thermosensitive color forming layer by heating, to injure the thermal head or cause so-called head stain resulting in poor printing quality. For this reason, silicic acid, urea resin pigment or pigments such as light calcium carbonate and the like having a high oil-absorbing property are used in many cases singly or as the admixture thereof, for the purpose of preventing the amount of the hand stain generated. However, as the amount of the pigment added increases, the adverse effects are resulted that a quantity of heat required for elevating to a temperature required enough for printing also increases and further such a reduction of the sensitivity as a color forming density decreases at the same energy applied occurs, etc. Furthermore, it cannot be said that the effect of preventing the head stain reaches a sufficient level yet.